1. Field of the Invention
The present invention relates to a displacement sensor for detecting the degree of displacement of an object that is subjected to force and thus displaced. If the degree of displacement of the object is detected, it is possible to measure the force applied to the object, or to measure the acceleration, angular acceleration, pressure, external force, etc. that created this force. The displacement sensor referred to here is a sensor that, by measuring physical value that changes depending on the degree of displacement, is capable of directly detecting the degree of displacement, and of indirectly measuring acceleration, angular acceleration, pressure, external force, etc.
In the present specification, a displacement sensor is described that utilizes the phenomenon that mass (an object that has mass) is displaced in response to acceleration. The displacement sensor measures, from the degree of displacement of the mass, the acceleration which has been applied to the mass. However, if the displacement sensor utilizing the technique set forth in the present specification is used in an environment in which the mass is displaced by means of angular acceleration, pressure, external force, etc., the displacement sensor can also measure the angular acceleration, pressure, external force, etc. applied to the mass. Furthermore, the displacement sensor that utilizes the technique of the present invention can also be utilized to measure the degree of displacement of the mass itself.
2. Description of the Related Art
An acceleration sensor has been developed that contains a mass that is displaced when acceleration is applied thereto, and that contains a condenser. One of electrodes of the condenser is formed in the mass. When acceleration is applied to the acceleration sensor, the mass is displaced in response to this acceleration, distance between the electrodes of the condenser changes, and the electrostatic capacity of the condenser changes. The acceleration applied to the acceleration sensor can be measured by measuring the electrostatic capacity of the condenser. An example of such an acceleration sensor is set forth in Japanese Laid Open Patent Application Publication H02 (1990)-134570.
The acceleration sensor of Japanese Laid Open Patent Application Publication H02 (1990)-134570 is provided with a mass, a beam supporting the mass, and two silicon sheets disposed above and below the mass and separated therefrom by a determined distance. The beam is adjusted so as to have adequate rigidity. Electrodes are formed at an upper face and a lower face of the mass. The electrode formed at the upper face of the mass is opposite the upper silicon sheet, and forms a first condenser. The electrode formed at the lower face of the mass is opposite the lower silicon sheet, and forms a second condenser. A pair of condensers is thus disposed in an upper and lower direction of the mass. The area of the electrode comprising the first condenser is identical with that of the electrode comprising the second condenser. Further, the distance between the electrodes of the first condenser is identical with the distance between the electrodes of the second condenser while acceleration is not applied to the mass.
When downwards acceleration is applied to the acceleration sensor, the mass is displaced upwards to a position of equilibrium by inertial force exerted upon the mass and by recovery force of the beam. When the mass is displaced upwards, the distance between the electrodes of the first condenser decreases, and the distance between the electrodes of the second condenser increases. Consequently, the electrostatic capacity of the first condenser increases, and the electrostatic capacity of the second condenser diminishes. The degree by which the electrostatic capacity of the first condenser increases is identical to the degree by which the electrostatic capacity of the second condenser diminishes.
As shown in FIG. 32, when acceleration is not being applied to the mass, that is, when the mass is in a standard location, the electrostatic capacity of both condensers is C0. If the mass has been displaced due to acceleration being applied thereto, and the electrostatic capacity of one of the condensers consequently changes to C0+ΔC, the electrostatic capacity of the other condenser changes to C0−ΔC. By calculating the difference between the electrostatic capacity of both condensers ((C0+ΔC)−(C0−ΔC)=2ΔC), it is possible to detect only the degree of change of electrostatic capacity ΔC in response to the acceleration. If the sensor is a differential sensor that outputs a quantity equivalent to the difference between the electrostatic capacity of both condensers, it is possible to exclude the standard electrostatic capacity C0 from the sensor output, and a sensor can be obtained that outputs the quantity 2ΔC, this being double the quantity of change.